Congestive heart failure results from the inability of the heart to pump blood throughout the body at its normal pace, causing blood to flow a slower rate with increased pressure. As a result, the heart is unable to meet the oxygen and nutrient demands of an individual's vital organs. Heart failure may be caused by cardiomyopathy, heart valves damage, coronary heart disease, hypertension, and, in some cases, diabetes. Worldwide, more than a million patients currently suffer from congestive heart failure. In the United States alone, thousands of patients with congestive heart failure are candidates for heart transplantation or an electro-mechanical heart implant, such as a ventricular assist device.
Ventricular assist devices (VAD) are implantable electro-mechanical pumps that are used to partially or completely replace the function of a failing heart. Ventricular assist devices do not replace the heart entirely, but rather assist the right (RVAD) or left (LVAD) ventricle in their ability to pump blood. The choice of the device depends on the underlying heart disease and the pulmonary arterial resistances, which determines the load on the right ventricle. LVADs are more common, as RVADS are typically only necessary when pulmonary arterial resistance is very high.
VADs require a power source to operate the pump, and thus require connecting the VAD to an external power source. Traditionally, the VAD is directly connected to an external power source by a transcutaneous power line. The transcutaneous power line requires an exit site in the abdomen that provides a portal of entry for pathogens, and thus often results in device related infections.
As an alternative to transcutaneous power lines, wireless energy transfer systems were developed to deliver power to a VAD across an unbroken skin layer, thereby eliminating the possibility of infection associated with power lines. Wireless energy transfer systems include transcutaneous energy transfer systems (TET) and coplanar energy transfer (CET) systems. Both TET and CET systems include an external transmitter coil coupled to a power source that wirelessly delivers energy to an implanted receiver coil. The implanted receiver coil is connected to the implant and relays the received energy to the implant. The systems differ, in part, by how the transmitter and receiver coils are arranged with respect to each other. In TET systems, the transmitter coil located on the surface of the skin and is parallel to (separated by a distance z) and coaxial with the implanted receiver coil. In CET systems, the transmitter coil surrounds a part of a body (e.g. placed within a belt around the chest) and is coaxial with a receiver coil positioned within that part of the body. In this manner, coils of CET systems are not separated by a distance z (z=0) but are positioned within the same plane (i.e. coplanar).
The configurations of both TET and CET systems are essential to the efficiency of the power transfer from the transmitter coil to the receiver coil. For example, when the ratio of the distance z to the diameter D1 of the transmitter coil is greater than 0.1, the efficiency of the power transfer decreases. As such, wireless transfer systems are very sensitive to any misalignment and movement of the coils (which increases distance z). In addition, the efficiency of the power transfer decreases when the ratio of the diameter D2 of the receiver coil to the diameter D1 of the transmitter coil is greater than 1. Accordingly, the transmitter coil of current TET and CET systems must be in close proximity with and have a diameter similar to the receiver coil in order to provide efficient and safe energy transfer. This requires that a patient wear the transmitter coil coupled to the power source on his/her person at all times. Unfortunately, the power source can be bulky and uncomfortable, and wearing the transmitter coil reduces but does not always prevent axial and radial misalignment problems. Since misalignment may ultimately result in loss of power to the VAD, there is a need for improved wireless energy transfer systems.